1. Field of the Invention
The present invention relates generally to lasers, and in particular, to semiconductor lasers subjected to modulation.
2. Description of Related Art
Semiconductor lasers have widespread uses across a variety of fields, such as optical data storage, laser pointers, medical sensing and diagnostics, and telecommunications. The relatively low voltage requirements, small size, and reduced cost make semiconductor lasers an ideal replacement for gas lasers in many applications.
Optical feedback may significantly affect the performance of a semiconductor laser. If a fraction of the light exiting a semiconductor laser cavity is reflected back into the cavity, the effects may be undesirable, leading to an increase in noise, chaotic operation, or broadened linewidth. Generally, an optical system is designed to reduce optical feedback wherever possible. For example, fibers and gradient index lenses used in a fiber-based telecommunications system have the ends polished at an angle, so that any light reflected off the polished ends is directed away from the laser.
Under certain conditions, optical feedback is deliberately used to affect the laser performance. For example, a diffraction grating is incorporated into a laser cavity to provide feedback only at a particular desirable wavelength. The laser uses this feedback to stabilize its performance with respect to external variables, such as variations in temperature. Although this example uses an optical element placed inside the optical cavity, a suitable design may alternatively use an optical element external to the laser cavity to stabilize performance.
A common and often undesirable feature of a laser is called chirp, which is a change in wavelength in the presence of intensity modulation. Chirp is observed most commonly in the output of pulsed lasers, where the leading and trailing edges of the pulse (in time) may have different wavelengths. Chirp may generally be found whenever the stimulated emission rate within the laser cavity is changed in order to change the emission intensity. Many applications, such as optical data storage, routinely require switching among various predetermined output power levels. In most cases, it is desirable that a constant wavelength be maintained from the laser, even as the output power is varied from level to level. For example, in a telecommunications system in which the laser light is coupled into a fiber and propagated over a great distance in the fiber, the fiber has a naturally occurring dispersion, in which light of different wavelengths propagates at different speeds. If a series of chirped pulses is transmitted along the fiber, the shape of the pulses will distort as the beam propagates, leading to a degradation of the transmitted signal. Therefore, it is generally desirable that the chirp of the laser should be reduced.
A vertical cavity surface emitting laser (VCSEL) is a particularly desirable type of laser, compared to a more common edge-emitting laser, for primarily three reasons. First, a VCSEL has a circularly symmetric output beam, which couples efficiently into an optical fiber. In contrast, an edge-emitting laser has an asymmetric output beam, which inherently couples less efficiently into a fiber. Second, VCSELs are produced and tested in wafer form, in contrast with edge-emitters, which are tested one isolated unit at a time. VCSELs are therefore inherently less expensive than edge-emitters. Third, VCSELs have a shorter cavity length than comparable edge-emitters, and as a result, may be more readily adapted to wavelength stabilization techniques. A potential drawback to VCSELs is that their output power is generally less than edge-emitting lasers.
In U.S. Patent Application No. 2002/0159487 A1 (referred to as '487), a light source is disclosed for use in optical communications systems. In one aspect, a gain region defined by a first and second mirror is provided having a corresponding resonant mode, and an external cavity defined by a third mirror and the second mirror is also provided having a plurality of resonant modes. The second mirror is configured such that one of the external cavity resonant modes is selected. The laser preferably has wavelength precision sufficient to eliminate the need for an external wavelocker, and is capable of being directly modulated in an essentially chirp-free manner.
However, the '487 patent fails to specify the values of the numerous specific design parameters required to realize a functional system while avoiding the undesirable effect of noise enhancement and chaotic operation. The '487 patent discusses an external cavity feedback system that is used for producing a laser beam that has reduced chirp. Unfortunately, establishing suitable values for the various parameters to obtain properly operating VCSELs under a variety of conditions is problematic.
Although there are numerous examples in the prior art of VCSEL type devices with three mirror cavities applied, for purposes of increased power output capability, or for more scientific purposes of understanding the stability of VCSELs in the presence of external feedback, these devices do not have desirable chirp characteristics.